After the end of their useful life, offshore oil and gas wells need to be safely taken out of service and plugged permanently. In a group of wells associated with a given offshore platform, some may come to the end of their economic life and may be plugged, perhaps with a temporary plug. There will come a time, though, where an entire platform and its associated wells must be taken out of service. This is a long and costly exercise, with the permanent plugging of each well taking on average about 50-60 rig days. There may be as many as 40 wells in a single well bay of an offshore platform.
In order to decommission a platform, it is normally necessary to provide a large rig tower capable of handling the heavy equipment e.g. for milling well casing, along with many other installations and facilities to support the operation of the rig tower and associated equipment. These installations and facilities would normally include at least power, a supply of drilling mud and accommodation for the personnel involved in the decommissioning operation.
The rig and associated installations may be accommodated on a large decommissioning jack up rig. Typically, the rig tower itself would be located on a cantilever structure extending beyond the side of the jack up itself. The jack up would be positioned alongside the platform to be decommissioned, with the cantilever structure extending above and across the platform, so that the rig tower is positioned above the well bay of the platform. Prior to this, any production rig tower on the platform would have been removed in a separate operation. Once the decommissioning rig tower is in place over the well bay, the tower may be moved on the cantilever structure to bring it into line with a specific well to be plugged. The cantilever structure may alternatively be movable on the jack up, or X-Y movement of the tower may be achieved by a combination of movement of the cantilever and tower. Once one well is dealt with, the tower is moved into position over another well, and so on.
An alternative approach is to locate a modular decommissioning rig structure on the platform to be decommissioned. This involves considerable stripping away of equipment on the platform, and adaptation of the platform to receive the large modular rig structure. The modular rig will normally include all the facilities required to run the large rig tower and heavy equipment necessary for plugging wells, as with the jack up: power, mud supply, accommodation, etc. This means the modular rig is very large and will typically occupy all or most of the surface of the existing platform, or at least cover the entire well bay. As with the jack up, the modular rig or the rig tower of the modular rig are movable in X and Y directions to allow the tower to be positioned sequentially over each well in the well bay.